Sydney's marine life turning troppo as coral, other species head south

By Peter Hannam

September 4, 2018 — 5.45pm

Tropical corals have been identified off Sydney's northern beaches and are "absolutely proliferating", providing habitat for a range of other species typically found much further north, according to the diver who found them.

Josh Sear, an underwater naturalist and photographer, has watched branching corals - whose scientific name is Pocillopora aliciae - gain purchase on large sandstone boulders in the Cabbage Tree Bay area near Manly in the past couple of years.

Newcomers: A headband damselfish swims amid a hard coral species now found off waters near Sydney.

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So far, there is little sign that the newcomers have dislodged other species in the Sydney areas at least, although it has happened elsewhere along the east coast.

"The big boulders of sandstone are just perfect for corals," Professor Booth said, noting the area is known locally as "the Barrens". They're "not so barren now", he said.

The Cabbage Tree Bay area is a protected zone, and earmarked to form part of the NSW government's proposed marine park for the Sydney region.

A damselfish swims near corals off Manly, on Sydney's northern beaches.

Photo: John Sear

Reef stresses

Tropical species may be expanding their range with warmer waters but the same force - climate change - is also playing havoc where sea temperatures rise too high.

A paper out on Wednesday in Nature Communications found the mass bleaching that killed about half of the Great Barrier Reef's corals over two consecutive summers did not spare deeper reefs either.

The researchers, including Ove Hoegh-Guldberg and other Australian-based scientists, found a summer upwelling of cooler water had provided "thermal relief" for corals in 2016.

That upwelling, however, subsided, leading to severe bleaching of 40 per cent of the deep reefs and about 6 per cent mortality, the paper found.

While much less than the death rates of shallow-reef corals, the delayed impacts of the bleaching may have triggered more mortality after the surveys were done.

The researchers noted deeper corals could provide a source of larvae that can help the recovery of harder-hit shallow reefs. However, that aid would be limited since the deeper reefs are home to a relatively small proportion of species compared with those closer to the surface.